1. Field of the Invention
The present invention relates to a RFID tag device for transmitting identification information by radio, a tag identification device for receiving and identifying the identification information, and a radio communication system.
2. Related Art
An RFID (Radio Frequency Identify) tag allows the information to be transmitted or received without contact, and is very convenient for a user. The secure convenience is in a trade-off relation with the risk of privacy leakage of the user for the RFID tag.
For instance, if an RFID tag is attached on a purse, behavior of the purse's owner can be known based on a detection history of the RFID tag for the purse. Also, by associating the owner with the tag detected on the purse, the behavior of the owner can be kept pursued at high probability, even if the owner exchanges the purse.
To defend the user from the privacy leakage using the RFID tag, the following measures (1) to (3), for example, are thought.                (1) Kill Switch/Kill Command        (2) Faraday Cage        (3) Blocker Tag        
Kill Switch/Kill Command of (1) is a manner for physically or logically invalidating the tag. The user can be defended from the privacy leakage by invalidating tag by this manner, but may lose the convenience that is sure to be obtained with tag.
The Faraday Cage of (2) is a manner for containing the tag in the material which shields electric wave and invalidating the tag while containing the tag. Though the tag can be validated or invalidated according to the user's intention, the tag is applicable only in limited cases (e.g., the case where the object contained in the chest should not be read from the outside), because the tag itself is often attached on the intended commodity. The convenience obtained by the tag is considerably restricted by the above (2).
Blocker Tag of (3) is a technology proposed by RSA Laboratories (refer to A. Juels, R. L. Rivest, and M. Szydlo. The Blocker Tag: Selective Blocking of RFID Tags for Consumer Privacy, ACM Press. 2003). This technology can be applied in accordance with an ALOHA method (MIT Technical Report MIT-AUTOID-WH-014) that is one of the Anti-Collision methods of the RFID tag. The method has a function of disabling the ID to be read under certain conditions by employing the characteristics of Anti-Collision. On the other hand, when the user wants to read the tag information, it is required that the Blocker Tag is invalidated by keeping the Blocker Tag at a distance or turning off the switch. Though the Blocker Tag is more convenient than the Faraday Cage, it is insufficient as automatic recognition that is one of the purposes of the RFID tag.
As a technology having the similar purpose, the key encryption technology has been proposed (refer to Japanese Patent Application Laid-Open No. 2001-236325).
The technology as disclosed in the patent document 1 is disadvantageous on the aspect of cost, because a sufficient calculation capability is needed for the tag itself.